Smart guide show projected extension of another line when hovering
This is a basic feature in CAD programs. When you are using the pen or line tools, you need to be able to hover over the end point of a line, and then snap the cursor to that line's hypothetical extension, so you can start a new line on that extension, so that each point you make can be co-linear with another.
See this video: https://youtu.be/9RgRoOunCDs?t=18 and attached images
I can't say how many times (probably in the tens of thousands) in my career when I have had to drag out a construction line extension just so i can start a new line that is co-linear, or at least starts co-linear with an existing line, and then delete the construction line when i am done.
We can have somewhat similar using smart guides, as Line Extension, but only for a straight segment, connected to straight segment from both sides (and there is a specific bug report about that — https://illustrator.uservoice.com/forums/601447-illustrator-bugs/suggestions/41077123-straight-line-extension-does-not-work-if-the-adjac).
But yes, sure, we should have this projection type smart guides.
Violet C commented
Yas. This. Illustrator has Smart Guides, I appreciate that... But mostly, I work visually. I'd rather see how far I'm straying from the original than paying attention to the dX/dY value.
I think a kind of 'ghost' shape of the original will help. History of the original shape would be appreciated, but just the last form of it as a guide would be great, too.
Violet C commented
I'm trying to modify a letterform. I divide the shape and create some space in one part. But I want to maintain the straightness/angle of the side I'm adjusting. As I move an anchor point, I'm not sure if the angle is maintained. No guide easily helps me do this.
Yes, I could duplicate the shape for a shape guide, and place it behind the shape being modified, give the shape guide a different color. But I will have to do this every time I modify a shape.
Maybe the guide already exists - pls. correct me if this is the case. Thank you.